1. Field of the Invention
This invention relates generally to a detection system for detecting possible failure of a heater for heating a vehicle battery and, more particularly, to a detection system for determining whether a heater in a battery circuit on a vehicle cannot be used to heat a cooling fluid for a liquid cooled battery, but can be used as a pre-charge resistor for vehicle soft starts.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium ion, nickel metal hydride, lead acid, etc. A typical high voltage battery for an electric vehicle may include 196 battery cells providing about 400 volts of power. The battery can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
The high voltage battery in an electric vehicle is selectively coupled to the vehicle's high voltage bus by battery contactors. When the vehicle is shut off, the contactors are opened and the battery is disconnected from the high voltage bus. When the vehicle is switched on, the contactors are closed and the battery voltage is coupled to the high voltage bus.
Several other high voltage components are electrically coupled to the high voltage bus, including a traction motor inverter module (TPIM) that inverts the DC high voltage bus signal to an AC signal suitable for the AC propulsion motors in the vehicle. The TPIM and other modules and circuits coupled to the high voltage bus generally include a relatively large capacitor coupled across the positive and negative lines of the high voltage bus that filter bus voltage noise that may otherwise have a degrading effect on the performance of the module. However, as the battery contactors are being closed and the battery voltage is coupled to the high voltage bus lines, these capacitors act as a direct short across the bus lines until the capacitor has had an opportunity to charge, which is generally only a few milliseconds. This limited time direct short has a degrading effect on many of the electrical components in the system as a result of the high voltage, including the capacitor itself and the contactors, which limits their life.
In order to eliminate or reduce this current spike from the direct short at system start-up, it is known to provide a pre-charge resistor in the battery circuit that operates as a load to limit the current while the several capacitors are being charged. In other words, the pre-charge resistor pre-charges the vehicle's high voltage bus prior to closing the main bus contactors during vehicle start-up in order to avoid high in-rush current spikes that may otherwise damage the high voltage capacitors. In one particular design, a negative battery contactor is closed at start-up and the pre-charge resistor is coupled across the positive bus contactor, which remains open until the pre-charge function is completed.
It is well known in the industry that high temperatures can be detrimental to a vehicle battery, and that most types of battery packs produce heat when being discharged during vehicle driving operation. It is also known that the life of a lithium-ion battery pack is a function of both temperature and state-of-charge of the battery pack, where high temperatures may be detrimental to battery pack life if those temperatures occur when the battery pack is in a high state-of-charge. Therefore, electric vehicles may employ a thermal management system to maintain battery pack temperature at a certain temperature that is known to extend battery pack life. The battery thermal management system typically includes a cooling system through which a cooling fluid flows that is directed around the battery in some manner to draw heat away from the battery. An electric heater is generally provided somewhere within the cooling fluid that heats the cooling fluid to raise the temperature of the battery to an optimal temperature during cold operating conditions for better performance.
U.S. patent application Ser. No. 12/853,695, filed Aug. 10, 2010, titled Combined Heating and Pre-charging Function and Hardware for Propulsion Batteries, assigned to the assignee of this Application and herein incorporated by reference, discloses a battery circuit that employs a resistive element for heating the cooling fluid for a liquid cooled battery on a vehicle, where the resistor is also used as a pre-charge resistor for starting the vehicle.
There is a drawback with using a single resistor for both the battery heating function and the pre-charge function where if the resistor becomes damaged or otherwise fails, then the vehicle is unable to be started because it cannot perform the pre-charge function. For example, if the liquid cooling system on a vehicle ruptures or otherwise leaks, where the cooling fluid drains out, and the heating resistor is subsequently turned on, the resistor would likely fail because its temperature will increase to a damaging level as a result of not being within the cooling fluid. In this situation, the battery may otherwise be operational, but at a lower performance. However, since the pre-charge function is not operational, the vehicle will not be able to be started, possibly resulting in a walk-home condition.